1. Field of Invention
This invention relates to a method for preparing a flame retardant material and in particular to a method for preparing a melamine salt of pentaerypolyol phosphoric acid.
2. Related Art
Conventional processes for manufacturing flame retardant materials or flame retardant additive materials, applied for electric wire, connect wire, wiring board or hot melt glue, such as a polyolefin flame retardant material containing phosphorous, oxygen and carbon, are all involving chemical processes using a large amount of organic solvents as reaction medium and catalysts.
For example, in U.S. Pat. No. 6,833,467 to Ma issued Dec. 21, 2004 and in U.S. Pat. No. 6,737,526 to Ma issued May 18, 2004, a large amount of toluene solvent, together with pentaerythritol and phosphorus pentoxide reactants, was used to run the two step esterification followed by the next step of neutralization with melamine to form melamine salt of bis-(pentaerythritol phosphate) phosphoric acid and melamine salt of pentaerythritol phosphoric acid. Also used in the two-step esterification process is a metal halide catalyst.
The main concept of the above inventions is to introduce a ball milling mechanochemical manufacturing process. However, since a large amount of organic solvents is used in that reaction, the environment requirements are hard to satisfy. In order to satisfy the environment requirements, the costs for pollution control must increase and will thus lead to an economically unsuccessful production. Besides, the use of a catalyst will also increase impurities of the products.
U.S. Pat. No. 4,454,064 discloses a method for preparing pentaerythritol phosphate regarding a reaction between pentaerythritol and phosphorus oxychloride reactants in a dioxane solvent under a temperature of 75 to 125° C. This is a useful intermediate in the preparation of flame-retardant materials, polyurethanes, and plasticizers.
U.S. Pat. No. 4,478,998 discloses a method for preparing an amino-s-triazine salt containing phosphoric acid, which is represented by the following formula,
in which the X and X′ are ammonium groups, therefore the salt will be a melamine salt of bis-(pentaerythritol phosphate) phosphoric acid. The salt can be used as a flame retardant additive for some specific polymer compositions.
However, the preparing methods as mentioned above all involve the usage of a large amount of solvents and will generate hydrochloric acid vapor, for which a large amount of water is required to wash away HCl from the product mixture, and solution containing excess POCl3 (phosphorus oxychloride) during the reaction, which will hardly ever satisfy the environment requirement and will increase the cost when taking the expenses for the environment requirement in mind.